Slide 1 CS3: Lecture 8 Advanced recursion

Slide 2 Schedule Oct 10Advanced recursion Oct 17Number-spelling Miniproject Oct 24Higher order procedures Oct 31More HOF Lists! (instead of sentences and words) Nov 7Recursion (extra advanced) Nov 14Midterm #2 Nov 21The big project!

Slide 3 Next lab: Catch-up day

Slide 4 Number Spelling miniproject Read Simply Scheme, page 233, which has hints Another hint (principle): don't force "everything" into the recursion. –Special cases may be easier to handle before you send yourself into a recursion

Slide 5 Recursive patterns Most recursive functions that operate on a sentence fall into: –Mapping: square-all –Counting: count-vowels, count-evens –Finding: member, first-even –Filtering: keep-evens –Testing: all-even? –Combining: sum-evens

Slide 6 "Tail" recursions Accumulating recursions are sometimes called "Tail" recursions (by TAs, me, etc).

Slide 7 Tail recursions and efficiency A tail recursion has no combiner, so it can end as soon as a base case is reached –Compilers can do this efficiently An embedded recursion needs to combine up all the recursive steps to form the answer –The poor compiler has to remember everything

Slide 8 Tail or embedded? (1/3) (define (length sent) (if (empty? sent) 0 (+ 1 (length (bf sent)))))

Slide 9 Embedded! (length '(a b c d))  (+ 1 (length '(b c d))) (+ 1 (+ 1 (length '(c d)))) (+ 1 (+ 1 (+ 1 (length '(d))))) (+ 1 (+ 1 (+ 1 (+ 1 (length '()))))) (+ 1 (+ 1 (+ 1 (+ 1 0)))) (+ 1 (+ 1 (+ 1 1))) (+ 1 (+ 1 2)) (+ 1 3) 4

Slide 10 Tail or embedded? (2/3) (define (sent-max sent) (if (empty? sent) '() (sent-max-helper (bf sent) (first sent)))) (define (sent-max-helper sent max-so-far) (if (empty? sent) max-so-far (sent-max-helper (bf sent) (first sent))))

Slide 11 Tail or embedded? (3/3) (define (find-evens sent) (cond ((empty? sent) ;base case '() ) ((odd? (first sent)) ;rec case 1 (find-evens (bf sent)) ) (else ;rec case 2: even (se (first sent) (find-evens (bf sent))) ) ))

Slide 12 > (find-evens '( ))  (se 2 (se 4 (se 6 ())  (2 4 6) (se 2 (se 4 (se 6 () sent = ( ) sent = ( ) sent = ( ) sent = ( 5 6 ) sent = ( 6 ) sent = ( )

Slide 13 Advanced recursions (1/3) "when it does more than one thing at a time" Ones that traverse multiple sentences –E.g., mad-libs takes a sentence of replacement words [e.g., ‘(fat Henry three) ] and a sentence to mutate [e.g., ‘(I saw a * horse named * with * legs) ]

Slide 14 Advanced recursions (2/3) Recursions that have an inner and an outer recursion (no-vowels '(I like to type))  ("" lk t typ) (l33t '(I like to type))  (i 1i/<3 +0 +yP3) (strip-most-popular-letter '(cs3 is the best class))  (c3 i th bet cla) (occurs-in? 'abc 'abxcde)  #f

Slide 15 Advanced recursions (3/3) Tree recursion: multiple recursive calls in a single recursive step There are many, many others

Slide 16 Advanced recursion columns (C) r o w s (R) Pascal’s Triangle How many ways can you choose C things from R choices? Coefficients of the (x+y)^R: look in row R etc.

Slide 17 (define (pascal C R) (cond ((= C 0) 1) ;base case ((= C R) 1) ;base case (else ;tree recurse (+ (pascal C (- R 1)) (pascal (- C 1) (- R 1)) )))

Slide 18 > (pascal 2 5) (+ (pascal 2 5) (pascal 2 4) (pascal 1 4) (+ (pascal 2 3) (pascal 1 3) (pascal 0 3) (+ (pascal 2 2) (pascal 1 2) (pascal 0 2)  1 (pascal 1 2) (pascal 0 2)  1 (+ (pascal 1 1) (pascal 0 1)  1 (+ (pascal 1 1)  1 (pascal 0 1)  1 (+ (pascal 1 1)  1 (pascal 0 1)  1

Slide 19 pair-all Write pair-all, which takes a sentence of prefixes and a sentence of suffixes and returns a sentence pairing all prefixes to all suffixes. –(pair-all ‘(a b c) ‘(1 2 3))  (a1 b1 c1 a2 b2 c2 a3 b3 c3) –(pair-all ‘(spr s k) ‘(ite at ing ong))  (sprite sprat spring sprong site sat sing song kite kat king kong)

Slide 20 binary Write binary, a procedure to generate the possible binary numbers given n bits. (binary 1)  (0 1) (binary 2)  ( ) (binary 3)  ( )